When two sets of identical twins marry each other and have children, those children are legally cousins but genetically full siblings, sharing about 50% of their DNA.
Why Some Cousins Are Actually Genetic Siblings
Here's a mind-bending genetic twist: when two sets of identical twins marry each other and have children, those kids are cousins in name only. Genetically? They're full siblings.
The science is surprisingly straightforward. Identical twins share virtually 100% of their DNA—they're genetic clones. So when twin sisters marry twin brothers, each child inherits genes from parents who are genetically identical to the other couple. From a DNA perspective, it's as if one couple had all the children.
These special cousins are called quaternary twins, and they share approximately 50% of their DNA—exactly what you'd expect from full biological siblings. Regular first cousins? They share only about 12.5% of their DNA.
The Half-Sibling Scenario
Even more common is when just one set of identical twins has children. Say twin sisters each have a kid with different partners. Those children are legally first cousins, but genetically they're half-siblings, sharing around 25% of their DNA.
Why? Because the children share one parent who is genetically identical to their aunt. Your mom and her identical twin have the same genetic blueprint, so you share as much DNA with your aunt as you do with your mother—50%. Your cousin inherited 50% from that same genetic source.
Real-World Quaternary Twins
This isn't just theoretical. In 2022, identical twin sisters Brittany and Briana Salyers married identical twin brothers Josh and Jeremy Salyers in a joint ceremony. Both couples now have sons born just months apart—making the boys cousins on paper and genetic brothers in reality.
The families even live together in the same house. As Brittany explained, "Our kids are cousins, but genetically they're genetic siblings. So that's a very unique circumstance that doesn't happen very often."
Legal implications? There aren't really any. Birth certificates list the biological parents, inheritance laws follow legal relationships, and the children are recognized as cousins. But for medical history, genetic testing, and organ donation matching, these cousins are as compatible as full siblings.
Why This Matters
Beyond being a fascinating genetic quirk, this has practical applications:
- Medical research: Scientists use "children-of-twins" studies to separate genetic influences from environmental ones
- Genetic counseling: These families need to understand shared disease risks
- DNA testing: Ancestry tests might show these cousins as half-siblings or even full siblings
- Organ matching: Quaternary twins have the same donor compatibility as biological siblings
So while family trees map out legal relationships, DNA tells a different story. These cousins share holiday dinners and genetic code—making family reunions both a social event and a genetics conference.